Faces with flipped Normals

[acmoles]

Firstly, I'm really appreciating this project!

I've noticed under certain conditions (usually a very twisted source mesh) one or two side faces can flip, pointing the opposite way to all the others, so that the mesh appears to have a hole (if backfaces are not drawn).

I imagine this has something to do with the check here:
https://github.com/nicoptere/THREE.ShellModifier/blob/master/THREE.ShellModiifer.js#L207

Can you think of any way to tell when faces are flipped and unflip them?